1. Field of the Invention
The invention relates generally to a battery charger and a method of charging a rechargeable battery, specifically alkaline secondary battery, Ni--Cd (Nickel-Cadmium), Ni--H (Nickel-Hydrogen) and Ni--MeH (Nickel Metal-Hydride) batteries.
2. Description of Related Art
In recent years, a large number of portable electronic devices, i.e. cellular phones, two-way radios, laptop computers and camcorders have become available. Accordingly, it has become necessary to provide a battery that is in a continuous state of operational readiness. In this regard, it is preferable to utilize a rechargeable battery and a method of recharging the battery which avoids both under and overcharging.
The charging of batteries involves forcing electrical current through the battery, usually under some control of the current (e.g. constant current) and often with some voltage control as well (e.g. maximum voltage). While there is some need for controlling the rate of the charging process itself, the most important need for control is when the battery becomes fully charged. After this point, continued charging of the battery leads to undesirable and wasteful overcharge reactions. Overcharge reactions in vented cells result in electrolysis and loss of water that has to be replaced, while in sealed cells it creates pressure and heat since the recombination reactions of gases produced during overcharge are exothermic.
Ideally, the overcharge reactions that require higher voltage can be prevented simply by limiting the charging voltage to a certain value. This simple approach is, unfortunately, only partially successful with certain cell types, e.g. lead acid cells, vented NiCd cells, and sealed lithium ion cells.
Sealed cells capable of recombining the overcharge reaction products are able to tolerate overcharge at low rates because the pressures of by-product gases are low and the heat generation is slow enough for the heat to be easily dissipated and lost. As long as no electrolyte constituent is being lost, it is not critical to terminate charging. However, continuous overcharge even at a low rate reduces the cycle life of the cells.
Rapid charging, i.e. charging in less than one hour, presents much more of a challenge with both vented and sealed cells. The first problem results from the limited rate of charge distribution or equilibration within the electrode plates. Also, at higher charge rates, the overcharge reactions begin to appear at a lower fraction of full charge. When overcharge reactions appear, the current efficiency of the charge reactions decline and most of the columbic energy is wasted on the overcharge reactions. To complete the charging process under these conditions overcharge reactions are tolerated for a sufficiently long time. This process causes damage to the battery. The rapid heating of a battery during high rate overcharge cannot be avoided and may cause safe pressure to be exceeded, as well as cell venting.
U.S. Pat. No. 5,352,967 details some of the known methods of charging storage batteries. The methods disclosed therein focus on various techniques of determining proper charged termination and include: constant current mode; delta temperature/delta time mode (dT/dt); negative delta voltage charge mode (-.DELTA.V); positive delta voltage or delta voltage/delta time mode (dV/dt); pulse charge mode; and reflex mode. In the constant current charge mode, the battery is continuously overcharged with a low current. Although the expenditure for a constant current source is relatively low, the long charging time causes damage to the cell. In the constant charge mode, it is customary to restrict the charging time. Therefore, as soon as a predefined time has elapsed, the charging operation is terminated. Accordingly, since the constant current charge mode does not take into account the charge condition or chemical makeup of the cell under or overcharging of the cell can result.
In delta temperature/delta time charging (dT/dt), the charging current is switched off once a predetermined slope in the temperature versus time curve has been reached. This method can generate false termination signals. The charging process may be terminated prior to the battery being fully charged if preset value of dT/dt is too low, or conversely, if dT/dt is too large, the charging process may be terminated too late.
In the negative delta voltage charge mode, a negative slope in the charge curve (dV/dt&lt;0) which appears after complete charging of the battery is used to terminate charging. The batteries are charged from a constant current source, and the charging voltage rises steadily for as long as the cell is capable of converting the supplied energy into chemical energy. When the batteries are no longer capable of storing the supplied energy, the supplied energy is converted into heat, and the cell voltage drops. The drop in the cell voltage is used as an indicator to terminate charge. However, this method can only be used for those type of batteries permitting high-current charging. The charging method itself suffers from the fact that during the charge operation, surface effects may result in fluctuations in the battery voltage. These fluctuations may be erroneously interpreted as a signal to terminate charge. Therefore, premature break off of the charging operation is often seen when utilizing the negative delta voltage charge mode. Additionally, nickel-metal hydride batteries do not have the pronounced charging voltage curves seen in nicad batteries, and as a result, are often overcharged using the negative delta voltage charge mode.
In the positive delta voltage or delta voltage/delta time mode (dV/dt), the slope of the voltage charging curve is evaluated to determine when to terminate charging of the battery. Theoretically, the rise in the charging voltage decreases when the battery is near full charge. Utilizing the mathematical differential of the charge curve, the reduction in this rise in the charging voltage can be evaluated as the criteria for terminating charging. This method suffers from the fact that the difference in the rise may not be dramatic enough to cause termination of the charging at a proper time with ensuing overcharge of the battery. Additionally, due to fluctuation in the charging curve, this first order derivative charging method may terminate charging prematurely.
Pulse charging utilizes a high current charge followed by an interruption period. The interruption period allows the voltage of the battery to be determined during a currentless phase or under open circuit voltage conditions (OCV) in order to determine the open circuit voltage of the battery (V.sub.OCV). Charging is terminated when V.sub.OCV has reached a preset reference voltage (V.sub.REF). However, as discussed herein, the reference voltage has a degree of uncertainty, and the value often depends on the rate of charge and the battery's design criteria. This method does not take these factors in to consideration when determining termination of charging.
Reflex mode charging also monitors the V.sub.OCV, but follows the charging mode with a discharge period and determines and stores V.sub.MAX. This V.sub.MAX is used as a reference wherein charging is terminated when V.sub.OCV is equal to or greater than V.sub.MAX. Reflex mode charging necessitates discharge of the battery.
Each of the methods discussed above suffer from potential under or overuse charge of the battery or cell. As discussed herein, overcharging a battery is very dangerous. A more accurate determination of charge termination which has been described in the related art in a limited manner is the use of a second order derivative of the charge versus time curve (d.sup.2 V/dt.sup.2) to signal charge termination.
U.S. Pat. No. 5,477,125 proposes a method of recharging a battery which comprises the steps of periodically interrupting the charge current, sampling the resistance free voltage (V.sub.O) after a delay period, and determining point or points on the V.sub.O (t), dV.sub.O /dt and d.sup.2 V.sub.O /dt.sup.2 curve which indicate onset of overcharge to signal charge termination. The current measuring periods are repeated every 10 seconds, and a delay period after current interruption is used to allow the battery to reach an equilibrium.
According to U.S. Pat. No. 5,477,125, after identifying the onset of overcharge, a suitable V.sub.REF is chosen as a function of either one or a weighted average of two or more characteristic points of overcharge. V.sub.REF is chosen for example by increasing V.sub.O by a certain percentage point (minimum on dV.sub.O /dt curve) or by decreasing V.sub.O by a certain percentage (V.sub.O corresponding to either the maximum on d.sup.2 V.sub.O /dt.sup.2 curve). An educated guess as to the percentage of V.sub.REF (i.e. 98% of V.sub.O corresponding to maximum on d.sup.2 V.sub.O /dt curve or 95% of V.sub.O corresponding to maximum on dV.sub.O /dt curve) is used as the termination point. This guesswork can result in accidental overcharging or under charging of the battery and ultimately leads to poorer performance and cell cycle life of the battery. U.S. Pat. No. 5,477,125 states that the V.sub.REF is independent of rate of charge. However, the present disclosure indicates that this appears to be an overbroad or erroneous statement. Establishing V.sub.REF independent of the rate of charge will cause V.sub.REF to have a large degree of error. The type of crystal that forms on the nickel oxide electrode is sensitive to the rate of charge being applied to the battery. A high rate of charge results in .gamma.-NiOOH while a low rate of charge results in .beta.-NiOOH crystal formation. The difference between these two is 30-40 mv under open circuit voltage conditions. Additionally, the interval after charge interruption at which the voltage of the battery is measured is important due to the fact that the open circuit voltage is unsteady for some time after charge interruption. The fluctuation at different time intervals depends on the rate of charge, the battery design and charging temperature. The rate of charge, for example, may increase the potential of the nickel-oxide electrode by 60-80 mv due to the appearance of high valence NiO.sub.2 at the end of charge. Considering only those factors enumerated above, an error in determining V.sub.REF on the order of at least 90-120 mV can be made if V.sub.REF is determined independent of the rate of charge.